1. Field of the Invention
This invention relates to an image reading apparatus which is used in a copier, a facsimile apparatus or the like, and more particularly, to an image reading apparatus for reading an original mounted on an original mount with a sensor.
2. Description of the Related Art
In order to exactly reproduce an image of an original by a copier or the like, it is necessary to precisely read the image of the original. Factors affecting image reading. precision include reading resolution, reading magnification, synchronization between the leading edge of the original and the position to start reading the image printed on the original, the degree of inclination between a reference line for mounting the original and a reading line, and the like. These factors must be adjusted.
In an image reading apparatus using a solid-state image sensor, such as a CCD (charge-coupled device) or the like, most of these factors are generally adjusted in a production line when producing the device. A conventional method of such adjustment will be described with reference to FIGS. 6 through 8. In FIG. 6, optical elements for guiding an image of an original to a CCD 98 comprise a full-speed mirror unit 93, including a slit 91, a first mirror 92 and an illuminating device (not shown), a half-speed mirror unit 96, including a second mirror 94 and a third mirror 95, and an imaging unit 100, including an imaging lens 97 for forming the image of the original on the CCD 98 while reducing the image, the CCD 98 and a CCD driver 99. In the adjustment, attention must be paid to basic items necessary for optical elements, such as magnification, resolution and the like, orthogonality between an image reading line 0 and the scanning direction P of the full-speed mirror unit 93, parallelism between the image reading line 0 and the center line Q of the slit 91, coincidence between the center line Q of the slit 91 and the longitudinal axis R of the CCD 98 in the direction of the shorter side of the slit 91 via the respective optical elements (hereinafter termed xe2x80x9csynchronous adjustmentxe2x80x9d), and the like. In the method of adjustment in the production line, a jig chart 101 shown in FIG. 7 is held at the position of original-mount glass 104, and the image of the jig chart 101 is formed on the CCD 98 by projecting light onto the chart 101. Optical distortion is generally adjusted while processing respective output signals from the CCD 98 by a personal computer 102 and monitoring the processed signals on a monitor 103 so that resolution, magnification, synchronism and orthogonality are adjusted using portions A1, the interval between portions B1, tapered charts C1, and portions D1 on the jig chart 101, respectively.
However, even when the above-described factors have been adjusted in the production line, optical distortion from the original mount to the CCD is produced due to vibration produced while the apparatus is conveyed along the production line. Also, when the apparatus is mounted on a plane which is not flat, distortion is produced in the main body of the apparatus, thereby producing optical distortion from the original mount to the CCD.
In order to solve such problems, a method has been proposed in U.S. Pat. No. 5,283,698, issued Feb. 1, 1994, in which distortion in the main body of an apparatus is easily measured after the main body of the apparatus has been installed at a xe2x80x9clocation of usexe2x80x9d.
In the ""698 patent, there has been proposed, as shown in FIGS. 8a and 8b, to provide apertures 91a for detecting deviation in the relative positional relationship between the slit and the CCD at two end portions of the slit 91 in the longitudinal direction corresponding to portions outside the effective image region of the CCD. It is thereby possible to perform optical adjustment without using a jig chart when installing the main body of the apparatus.
In the invention described in the ""698 patent, however, it is necessary to precisely dispose a chart having the slit and the apertures, formed separately from the full-speed mirror unit 93, on the full-speed unit 93, thereby causing an increase in production cost due to adjustment and the like. If the slit is not precisely disposed, there is also the possibility of blocking image light from an original more than necessary. In addition, since portions outside the effective image region of the CCD are used, the CCD must have an additional number of pixels. Furthermore, since it is impossible to dispose the slit on the same surface as the upper surface of the original-mount glass, i.e., the surface of an object to be read, the apertures must be read at a defocused position of the imaging lens, thereby reducing reading accuracy.
The present invention has been made in consideration of the above-described problems.
It is an object of the present invention to provide an image reading apparatus capable of precisely measuring distortion in the main body of the apparatus.
It is another object of the present invention to provide an image reading apparatus capable of precisely measuring optical distortion from an original mount to an image-light reading sensor.
According to one aspect, the present invention, which achieves these objectives, relates to an image reading apparatus, comprising an original mount, scanning means for scanning an original mounted on the original mount, a sensor for reading an image scanned by the scanning means, and a reference image provided on substantially the same plane as an original-mounting surface of the original mount. The reference image is scanned by the scanning means and is read by the sensor. The apparatus further comprises measuring means for measuring distortion in a main body of the apparatus based on an output from the sensor obtained by reading the reference image.
According to another aspect of the present invention, an image forming apparatus for forming an image on a recording material in accordance with an image signal obtained by reading an original includes an image reading portion and an image recording unit. The image reading portion includes an original mount including an original-mounting surface, a scanner for scanning an original mounted on the original mount, a sensor for reading an image scanned by the scanner and outputting a signal indicative of the image scanned, a reference image provided in a plane substantially the same as the original-mounting surface of the original mount, the reference image being scanned by the scanner and being read by the sensor, and measuring means for measuring distortion in a main body of the image reading portion based on an output from the sensor obtained by reading the reference image. The image recording unit records the image scanned by the scanner on a recording medium using the signal output by the sensor.
According to another aspect of the present invention, a method of measuring distortion in a main body of an image reading apparatus includes the steps of scanning the reference image provided in a plane substantially the same as an original-mounting surface of an original mount in the image reading apparatus, reading the reference image scanned, outputting a signal in accordance with the reference image read, and measuring distortion in the main body of the image reading apparatus based on the signal output.
The foregoing and other objects, advantages and features of the present invention will become more apparent from the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings.